Carbon nanotubes have similar electrical conductivity to copper, have the same thermal conductivity as diamond (diamond's thermal conductivity is the best in natural world) and have 100 times higher strength than steel. Therefore, carbon nanotubes have attracted considerable attention as raw materials to be used for next-generation electronic devices, photoelectronic devices and circuits and the like.
Further, carbon nanotubes are made into a quasi-one-dimensional system because of high electrical and optical anisotropy that results from their large aspect ratios and small diameters. On the basis of this electrical and optical anisotropy, various recent efforts have been made to use carbon nanotube sheets as linear polarizers in a visible frequency range as well as in a long-wavelength range.
Meanwhile, a polarizer is an optical device for selectively transmitting linearly-polarized light in a predetermined direction from irregularly-mixed polarized light. That is, a polarizer serves to selectively absorb or refract linearly-polarized light in one direction to transmit only linear polarized light perpendicular to the direction. Here, the direction in which linearly-polarized light is transmitted is referred to as “a polarizing axis”. An ideal polarizer transmits all the components of a polarizing axis and blocks the components perpendicular to the polarizing axis.
As a transitional terahertz (THz) polarizer, a wire grid polarizer has dominated a polarizer market. A wire grid polarizer may be fabricated by mechanically winding a wire under high tension on a frame. Such a wire grid polarizer guarantees a low light loss and minimum light dispersion because it does not need a substrate. One of the factors necessary for fabricating the wire grid polarizer is winding tension. In the fabrication of the wire grid polarizer, it is important to uniformly maintain the diameters of metal wires and the intervals between metal wires. However, it is impossible to realize such conditions, and each independent metal wire assembly of this wire grid polarizer is often cut because the strength of metal wires is very low. Further, when the wire grid polarizer is exposed to air, the metal wire is rapidly oxidized, thus decreasing the lifespan of the wire grid polarizer. Meanwhile, the wire grid polarizer is not suitable to be used in a high terahertz frequency range because the conductivity of each wire is limited and the interval between wires must be smaller than the wavelength of incident beam to an extinction ratio.
In order to overcome the above problems, there was proposed a method of fabricating a wire grid polarizer by forming a wire grid on a substrate. The wire grid polarizer fabricated using this method is relatively strong and operates in a high terahertz frequency range because elaborate pitches can be made thanks to recently-developed nano-patterning technologies. Nevertheless, the substrate basically incurs the lows of light and the dispersion of incident waves, and the nano-patterning technologies require a time-consuming process.
Thus, a polarizer using carbon nanotubes has been proposed. However, this polarizer using carbon nanotubes is also problematic in that it exhibits a lower extinction ratio than a different type of polarizer such as a wire grid polarizer or a liquid crystal polarizer. Therefore, a means for solving the problem is eagerly required.